The invention relates to a process for making crosslinked polyvinylpyrrolidone. In particular, the invention is a productive way to make crosslinked polyvinylpyrrolidone that is easy to isolate and has a low swell volume.
Polyvinylpyrrolidones have diverse utility. They are used in polymer films, adhesives, hair and skin-care formulations, pharmaceutical tablet binders and disintegrants, and beverage clarifiers. Polyvinylpyrrolidones are normally produced by free-radical or base-catalyzed polymerization of N-vinylpyrrolidone (NVP). Initiators such as hydrogen peroxide or organic peroxides polymerize NVP to give polymers having relatively low molecular weight and a low degree of crosslinking. These products are commonly known as xe2x80x9cPVP K30xe2x80x9d and xe2x80x9cPVP K90.xe2x80x9d
In contrast, crosslinked polyvinylpyrrolidone (hereinafter called xe2x80x9ccrosslinked PVPxe2x80x9d or xe2x80x9cPVP-Pxe2x80x9d) has a high molecular weight and a high degree of crosslinking. It is produced by base-catalyzed polymerization of NVP. Crosslinked PVP is usually produced by one of two general methods. In one approach, NVP is polymerized in the presence of an added difunctional crosslinker such as divinylimidazolidone. In an alternative method, the crosslinker is generated xe2x80x9cin situxe2x80x9d by heating the reaction mixture at 130xc2x0 C. to 140xc2x0 C. in an initial stage. The temperature is then reduced to about 100xc2x0 C. After an xe2x80x9cinduction period,xe2x80x9d during which little or no polymerization occurs, the reaction rate increases rapidly, usually accompanied by an exotherm, and polymerization proceeds to completion. Unfortunately, the induction period can last for hours, which hampers productivity. Thus, an important challenge in making crosslinked PVP involves reducing or eliminating the induction period.
In PCT Int. Appl. WO 94/20555, Tseng teaches a process for making highly crosslinked PVP having a swell volume less than about 65 mL H2O/10 g polymer using an in situ-generated crosslinker. By performing the polymerization under an initial gas pressure of at least 2 bars (about 30 psig), Tseng was able to practically eliminate the induction period. Under similar conditions (described in U.S. Pat. No. 3,277,066), but without the added gas pressure, an induction period of 2-3 hours is typically observed.
While eliminating the induction period is valuable, Tseng""s process starts with a relatively high concentration of N-vinylpyrrolidone (75-85 wt. %) in the aqueous mixture. Consequently, the resulting crosslinked PVP mixture is highly concentrated, which makes the polymer difficult to isolate. Ideally, a more dilute solution of N-vinylpyrrolidone could be used to facilitate polymer isolation. Moreover, the polymer produced in this process usually has a swell volume of about 50-60 mL H2O/10 g polymer. For some applications, PVP-P having a somewhat lower swell volume (30-40 mL H2O/10 g polymer) is desirable.
U.S. Pat. No. 5,286,826 teaches a two-stage process for making highly crosslinked PVP having a low swell volume. A relatively concentrated aqueous NVP solution (80 wt. %) is heated in the presence of base under low pressure (0-3 psig) to generate an in situ crosslinker. Next, the solution is cooled to about 30xc2x0 C., and is diluted with water to reduce the NVP concentration to 5-30 wt. %. The mixture is then reheated to 100xc2x0 C., whereupon polymerization proceeds to give a product having a swell volume of less than 45 mL H2O/10 g polymer. The process is time-consuming because of the need for cooling, diluting with water, and reheating after generating the crosslinker.
Still needed is a productive process for making crosslinked PVP. Preferably, the induction period for making the PVP-P would be minimized or eliminated. A valuable process would operate at relatively low concentrations of N-vinylpyrrolidone to facilitate polymer isolation. Preferably, the process would avoid the need for a water dilution step. Ideally, the process would give crosslinked PVP having a low swell volume.
The invention is a process for making crosslinked polyvinylpyrrolidone. The process comprises two steps. In a first step, an aqueous mixture that contains from about 65 to about 70 wt. % of N-vinylpyrrolidone and from about 1.5 to about 6.0 mole % of an alkali metal hydroxide (based on the amount of N-vinylpyrrolidone) is heated under added inert gas pressure. This heating step, which generates a crosslinker in situ, occurs in a sealed reactor at a temperature within the range of about 130xc2x0 C. to about 150xc2x0 C. at an ultimate reactor pressure of at least about 55 psig. In a second step, the reactor temperature is reduced to a value within the range of about 95xc2x0 C. to about 105xc2x0 C. to initiate polymerization.
The two-step process is an easy and productive way to make crosslinked polyvinylpyrrolidone. Applying pressure in the first step effectively reduces or eliminates the induction period. Surprisingly, a relatively dilute aqueous solution of N-vinylpyrrolidone can be used, so the polymer product is exceptionally easy to isolate and purify. The resulting PVP-P has a swell volume less than about 40 mL H2O/10 g polymer, which makes it particularly valuable for beverage clarification.
The invention is a two-step process for making crosslinked polyvinylpyrrolidone by base-catalyzed polymerization of N-vinylpyrrolidone. As used herein, xe2x80x9ccrosslinked polyvinylpyrrolidone,xe2x80x9d xe2x80x9ccrosslinked PVP,xe2x80x9d and xe2x80x9cPVP-Pxe2x80x9d refer to polymers of N-vinylpyrrolidone that are highly crosslinked and insoluble in water. Preferably, N-vinylpyrrolidone is the only monomer used. However, minor amounts (up to about 10 mole %) of other ethylenic monomers, including other N-vinyl monomers, can be included.
Step One: Generation of an xe2x80x9cIn Situxe2x80x9d Crosslinker.
In the first step, an aqueous mixture that contains N-vinylpyrrolidone (NVP) and an alkali metal hydroxide is heated under added inert gas pressure. The aqueous mixture contains from about 65 to about 70 wt. % of NVP. If the NVP concentration is significantly less than about 65 wt. %, the induction period is too long (see Comparative Example 3, below) or polymerization does not occur (see Comparative Example 4). On the other hand, if the NVP concentration is much greater than about 70 wt. %, the polymer product is too concentrated and is more difficult to isolate and purify. Moreover, when the NVP concentration is as high as 80 wt. %, the swell volume of the polymer is usually significantly greater than 40 mL H2O/10 g polymer (see Comparative Examples 7 and 8).
The aqueous mixture contains an alkali metal hydroxide. Suitable alkali metal hydroxides include sodium hydroxide, lithium hydroxide, potassium hydroxide, cesium hydroxide, and the like. Potassium hydroxide and sodium hydroxide are economical and effective, and are therefore preferred. Sodium hydroxide is particularly preferred.
The amount of alkali metal hydroxide used is within the range of about 1.5 to about 6.0 mole % based on the amount of N-vinylpyrrolidone used. For example, 2.5 mole % of sodium hydroxide would be 0.025 moles of sodium hydroxide for every mole of N-vinylpyrrolidone used. A more preferred range is from about 2.0 to about 4.0 mole %. If the concentration of alkali metal hydroxide is too low, it is often difficult to achieve a swell volume that is less than 40 mL H2O/10 g polymer (see Comparative Example 5). Most preferably, about 2.8 mole % of sodium hydroxide is used.
The reaction temperature in the first step is within the range of about 130xc2x0 C. to about 150xc2x0 C., preferably from about 135xc2x0 C. to about 145xc2x0 C. Most preferably, the reaction temperature is about 140xc2x0 C. The reaction mixture is held at this temperature for a time effective to generate xe2x80x9cin situxe2x80x9d a difunctional crosslinker mixture, which typically includes mostly ethylidene vinylpyrrolidone (EVP) and a small fraction of ethylidene bis(vinylpyrrolidone) (EBVP). The in situ-generated crosslinkers act as starters for the polymerization, which occurs in step 2. The polymerization is sometimes called xe2x80x9cpopcornxe2x80x9d polymerization because of the apparition of solid puffs of PVP-P particles as polymerization proceeds.
In the first step, the aqueous mixture is heated under added inert gas pressure in a sealed reactor. Without any added gas pressure, heating the mixture to the desired temperature (about 130-150xc2x0 C.) generates autogenous reactor pressure of about 50 psig. In the process of the invention, the reactor is pressurized with an inert gas such as nitrogen or argon, preferably to an ultimate pressure (at 130-150xc2x0 C.) greater than about 55 psig, and more preferably within the range of about 60 psig to about 100 psig. Most preferably, the ultimate reactor pressure is within the range of about 60 psig to about 80 psig. Interestingly, a relatively small increase in the pressure above the reaction mixture provides a significant benefit in reducing or eliminating the induction period. In the absence of added gas pressure, we observed an undesirably long induction period of several hours (see Comparative Example 6).
We found that the timing of applying the added inert gas pressure is not crucial. For instance, the pressure can be applied before (see Example 2) or after (see Example 1) heating the aqueous mixture of NVP to the intial reaction temperature of 130-150xc2x0 C. Either way, the product is easy to isolate and has a desirably low swell volume.
Step Two: Polymerization.
In the second step, the reactor temperature is reduced to a value within the range of about 95xc2x0 C. to about 105xc2x0 C., more preferably at about 100xc2x0 C., to initiate polymerization of the remaining N-vinylpyrrolidone. Normally, the temperature is allowed to drop to the 95-105xc2x0 C. range, and is then regulated at that temperature with heating or cooling as needed. Usually, an exotherm occurs immediately or soon after the temperature is dropped to the 95-105xc2x0 C. range, indicating that polymerization is underway. The reaction mixture is held at 95-105xc2x0 C. until most of the NVP polymerizes. The reaction is normally complete within about 5-6 hours.
After the polymerization is complete, the crosslinked PVP is isolated by any suitable technique. Usually, water is added, the mixture is stirred for awhile, and the aqueous phase is separated from the polymer by decanting, filtering, or the like. The polymer is preferably washed with additional water to remove residual alkali metal hydroxide. If desired, the pH of the washings can be monitored until a neutral filtrate is observed. The polymer is then normally dried at room or elevated temperature under atmospheric or reduced pressure. Preferably, elevated temperature and reduced pressure are used to drive off the residual water.
Crosslinked PVP prepared by the process of the invention has a relatively low swell volume. In particular, it has a swell volume less than about 40 mL H2O/10 g polymer. Preferably, the PVP-P has a swell volume within the range of about 34 to about 36 mL H2O/10 g polymer. Crosslinked PVP having a swell volume less than about 40 mL H2O/10 g polymer is valuable for applications that require limited swelling of the polymer. While some adsorptivity is needed, too much adsorption can result in disintegration of the polymer. Disintegration is desirable for pharmaceutical tablets, but it is undesirable in beverage clarification, where a solid filter cake needs to remain intact during the beverage filtering process to prevent impurity breakthrough. Moreover, PVP-P having limited swell volume minimizes the volume of solid filter cake generated, thereby reducing waste disposal costs.
Crosslinked PVP produced using the process of the invention preferably has a particle size within the range of about 250 to about 350 microns. It preferably has a glass-transition temperature (Tg), measured by differential scanning calorimetry, within the range of about 185xc2x0 C. to about 190xc2x0 C.; more preferably, the Tg is about 185xc2x0 C.